1. Field of the Invention
The present invention relates to a unit and a method for driving a semiconductor device having a switching function. More specifically, the invention relates to a semiconductor device driving unit and method which can reduce a surge voltage while suppressing increase of a switching loss when the semiconductor device is switched.
2. Related Art
Conventionally, electric vehicles generally employ a synchronous motor which is driven by three-phase AC voltages. Therefore, electric vehicles incorporate an inverter which drives the synchronous motor by converting a DC output voltage of a battery (DC power source) into three-phase AC voltages. An inverter that is incorporated in an electric vehicle is expressly called an electric vehicle inverter.
Many electric vehicle inverters employ PWM (pulse width modulation) control and employ an IGBT (insulated gate bipolar transistor) as a power semiconductor device for realizing the PWM control (refer to Patent documents 1-3).    [Patent document 1] JP-A-2007-306166    [Patent document 2] JP-A-2008-078816    [Patent document 3] US 2010/0008113
IGBTs are self-turn-off semiconductor devices which are driven by a gate-emitter voltage Vge and can be turned on and off by a gate input signal.
The term “turn-off switching” means switching from a collector-emitter conductive state to a collector-emitter cutoff state in an IGBT. The term. “turn-on switching” means switching from a collector-emitter cutoff state to a collector-emitter conductive state in an IGBT.
In electric vehicle inverters, an FWD (free wheel diode) is paired with such an IGBT. That is, the FWD is a bypass diode for the IGBT and is connected to the IGBT in parallel in the opposite direction to the input/output direction of the IGBT.
Electric vehicle inverters are equipped with a circuit (hereinafter referred to as a semiconductor device drive circuit) for driving an IGBT. The semiconductor device drive circuit controls turn-on and turn-off of the IGBT by varying the gate-emitter voltage Vge of the IGBT.
However, a surge voltage occurs in a transient period of switching such as turn-on or turn-off of the IGBT. The surge voltage will be outlined below.
A circuit (bus) to which an IGBT is connected has a stray inductance, which gives inertia to a current, that is, acts so as to obstruct variation of the current. Therefore, when a current is going to decrease rapidly, an electromotive force is induced across the stray inductance in such a direction as to obstruct decrease of the current. That is, in electric vehicle inverters, the electromotive force is induced in such a direction as to be added to the battery power source voltage. A voltage that is generated on the basis of an electromotive force generated in this manner is called a surge voltage.
In electric vehicle inverters, plural units (e.g., three units) of IGBTs (each unit consists of two series-connected IGBTs) are parallel-connected to a three-phase load of a synchronous motor. In each unit of IGBTs, when one IGBT is turned on, the other IGBT is turned off. Therefore, in a switching transient period of one unit, the collector current of one IGBT decreases rapidly, whereby a large surge voltage is induced and superimposed on the power source voltage. A resulting voltage is applied between the collector and the emitter of the IGBT.
IGBTs are required to have such a device breakdown voltage as to withstand such a surge voltage. Therefore, naturally, as the surge voltage becomes higher, the required breakdown voltage increases and IGBTs become larger. Industrial inverters used in plants etc. can employ large IGBTs because sufficient installation spaces exist in factories. However, in the case of electric vehicle inverters, it is difficult secure a wide installation space in an electric vehicle and hence it is very difficult to employ large IGBTs.
As such, IGBTs to be incorporated in electric vehicle inverters are required to be small in size. IGBTs can be miniaturized by reducing the device breakdown voltage, which is realized by reducing the surge voltage.
Since as described above the surge voltage is generated by a rapid current decrease, the surge voltage can be reduced by decreasing the rate of a current decrease. That is, the surge voltage can be reduced by lowering the switching speed of the IGBT, that is, shortening the current/voltage rise time and fall time of switching of the IGBT.
However, lowering the switching speed to reduce the surge voltage results in increase in the loss (hereinafter referred to as a switching loss) of the IGBT and the FWD in a switching transient period.
On the other hand, as described above, the surge voltage is increased if the switching speed is increased to lower the switching loss.
As described above, a tradeoff relationship exists between the surge voltage and the switching loss. This relationship will be hereinafter referred to as a “tradeoff characteristic of the surge voltage and the switching loss.”
Thus, in electric vehicle inverters, it is now demanded to improve the tradeoff characteristic of the surge voltage and the switching loss, in other words, to reduce the surge voltage while suppressing increase of the switching loss at the time of switching of IGBTs.
Although patent documents 1-3 disclose several techniques for satisfying this demand, it cannot be said that any of these techniques are satisfactory. A new technique capable of fully satisfying the demand is now desired.
Although the above description has been directed to electric vehicle inverters, electric vehicle inverters are not the only devices that are required to be miniaturized; various devices using semiconductor devices having a switching function are required to be miniaturized. Therefore, a new technique capable of fully satisfying the above demand is required to be not only applicable to IGBTs for electric vehicle inverters but also broadly applicable to general semiconductor devices having a switching function.